This project was designed to determine whether persistent or tolerant infections are associated with chronic diseases such as ALS, MS, PML, Bell's palsy, Parkinson's disease, etc. It proposes to purify and concentrate SSPE virus to examine the biochemical and antigenic properties of this virus in comparison with the conventional measles virus. It also proposes to conduct epidemiological surveys concerning the occurrence of SSPE in the United States. Brain tissue from patients with encephalopathies will be examined by electron microscopy, fluorescent microscopy, and whenever possible, cultured in vitro. Brain cultures will be submitted to virologic and serologic assays, inoculated into experimental animals, and cocultivated with a variety of indicator cell systems in efforts to isolate and identify intracellular agents. Molecular hybridization studies with brain tissue will be conducted to search for virus "fingerprints" such as messenger RNA, virus specific DNA and RNA.